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Stratovolcano: Definition, Formation & Facts

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  • 0:00 What Are Stratovolcanoes?
  • 1:10 What Happens In A Volcano?
  • 3:15 Eruption Types At…
  • 5:30 Lesson Summary
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Lesson Transcript
Instructor: Amy Lange

Amy has taught university-level earth science courses and has a PhD in Geology.

This lesson covers one of the most dynamic natural features on earth - the stratovolcano. In this lesson, learn how they form, what makes them tick, and the eruptions they produce.

What are Stratovolcanoes?

If asked to draw a volcano, you'd probably draw a steep-sided mountain shaped like a triangle, maybe with steam coming from the top. This iconic shape that we commonly think of as a 'volcano' is actually a stratovolcano. A stratovolcano is a conical-shaped volcano composed of steeply-dipping layers of lava, hardened ash, and other material erupted from the main volcanic vent. Stratovolcanoes are also commonly called composite volcanoes. Mt. Fuji in Japan is an example of a stratovolcano.

Stratovolcanoes are most found commonly along subduction zones, which are boundaries between two tectonic plates where an oceanic plate is sinking into the mantle beneath another tectonic plate. These stratovolcanoes appear as chains on the upper tectonic plate, like the Aleutian Islands in Alaska and the Cascade volcanoes in the Pacific Northwest.

The Ring of Fire is an almost continuous chain of stratovolcanoes surrounding the Pacific Ocean. The stratovolcanoes along Asia and Alaska are due to the subduction of the Pacific plate. The stratovolcanoes along the Americas are due to the subduction of smaller tectonic plates.

What Happens in a Volcano?

In grade school, many times science projects include a simulated volcanic eruption using baking soda and vinegar. In this simple experiment, pour baking soda into the model of a volcano and add vinegar. The reaction between the baking soda and vinegar produces carbon dioxide gas. This gas expands and escapes out of the top of the volcano, bringing the remaining liquid with it as well, much like opening a soda that has been shaken. Just like in this experiment, actual volcanic eruptions are driven by gases as well.

Magma beneath a volcano is made of liquid rock, gases, and possibly crystals. The gases are trapped in the magma by the high pressures deep in the earth. As magma moves toward the surface, the pressure decreases, allowing the gas to escape. A gas will expand to the size of its container. So once gas is released from the magma at lower pressure, it immediately expands, and this expansion creates a tremendous amount of pressure on the magma and the surrounding rock. If the pressure becomes too great, an explosive eruption occurs.

This illustration shows an idealized cross section of a stratovolcano. Notice the steeply dipping beds making up the flanks of the volcano. The crater at the top marks the central vent where eruptions occur.
Stratovolcano cross section

Stratovolcanoes may have additional smaller vents on their flanks from which minor eruptions can occur. Stratovolcanoes are characterized by explosive eruptions due to their high gas content. This is in contrast to shield volcanoes, like Mauna Loa in Hawaii, which have gently dipping sides and mild eruptions. The deposits that make up stratovolcanoes evidence this explosive history because of the tephra left behind. Tephra is the term for any material produced by a volcanic eruption. Tephra includes volcanic ash, small cinders, and even pumice. The hardened rock deposit is called a tuff.

This photo shows a large tuff deposit called the Bishop Tuff. Tuffs are made of bits of tephra ejected during an eruption. The size of this tuff tells us that it was produced during a massive eruption.
Tuff image

Eruption Types at Stratovolcanoes

As discussed, stratovolcanoes are the sites of explosive eruptions. The most common eruption types at stratovolcanoes are Strombolian, Vulcanian, and Plinian eruptions.

Strombolian eruptions, named after the Stromboli volcano in Italy, are relatively minor eruptions of lava and tephra lasting for a brief time. When viewed at night, they look like a glowing lava fountain at the top of the volcano. Strombolian eruptions usually occur in sets at regular intervals between the eruptions. Strombolian eruptions will normally produce small lava flows as well.

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